1. Field of the Invention
The present invention relates to a deposited film forming apparatus in which a deposited film, in particular, a functional deposited film, especially, an amorphous semiconductor film capable of using in a semiconductor device, a light-receiving member for electrophotography, a line sensor for inputting an image, an image pickup device, a photovoltaic device, or the like is formed on a cylindrical conductive substrate by plasma CVD, and it also relates to an electrode for use in the apparatus.
2. Related Background Art
As materials useful for semiconductor devices including thin film transistors, light-receiving members for electrophotography, line sensors for inputting an image, image pickup devices, photovoltaic devices, or various types of electronics elements, non-single-crystal materials, in particular, amorphous materials such as amorphous silicon (e.g., amorphous silicon compensated by hydrogen or/and a halogen), amorphous silicon nitrides, amorphous silicon carbides, and amorphous silicon oxides are known. Some of the semiconductor devices prepared by utilizing a deposited film comprising the materials as mentioned above are practically used.
However, among these devices, there are some points to be improved in production of devices, characteristics of a deposited film, costs in production, or the like. For example, when a light-receiving member (photosensitive member) for electrophotography is to be manufactured, since the surface of a substrate for the photosensitive member has a relatively large area, it requires that a deposited film is uniformly formed in a thickness and electrical characteristics with a small number of film defects over the substrate. Further, a deposited film forming apparatus which can assure sufficient productivity and yield is demanded in order to satisfy a requirement of higher characteristics in recent years.
One example of an apparatus for forming such a deposited film is shown in FIGS. 1A and 1B. FIG. 1A is a transverse sectional view illustrating the deposited film forming apparatus. FIG. 1B is a longitudinal sectional view illustrating the deposited film forming apparatus.
In FIGS. 1A and 1B, reference numeral 700 denotes a reaction vessel. The reaction vessel 700 is connected to an exhausting apparatus (not shown) capable of exhausting a gas and the like inside the vessel through an exhaust pipe 705. A source gas for film formation such as a silane gas, a methane gas, a diborane gas, a phosphine gas or the like is supplied from a source gas supply pipe 703 connected to a source gas supply system (not shown) constituted by a bomb, a pressure adjuster, a mass-flow controller, a valve, and the like into a space (internal chamber) 702 surrounded by substrates 701. (In FIG. 1A, eight cylindrical substrates 701 are arranged at an equal distance on the same circumference.)
An exhaust amount is adjusted to generally set a gas pressure in the space 702 to be a desired pressure of 1.times.10.sup.-2 to 1,000 Pa.
A microwave power source (not shown) supplies a microwave power of, e.g., 2.45 GHz, into the space 702 through an isolator (not shown), a wave guide 707, and a microwave supply window 706 to generate glow discharge in the space 702, thereby forming a deposited film on each substrate 701. The substrate 701 is supported by a rotating shaft 708 through a holder (not shown). The rotating shaft is connected to a motor 709 through a gear 710. The substrate is rotated by the motor to form a uniform deposited film on the substrate 701.
The substrate 701 is heated by a heater 704 to a temperature of, e.g., 100.degree. C. to 400.degree. C., required to form a deposited film.
As a deposited film forming apparatus in which the above problems are solved, a deposited film forming apparatus by a plasma CVD method using a microwave is disclosed in U.S. Pat. No. 5,129,359. They disclose a deposited film forming apparatus having the following arrangement. A substrate, a microwave supply means, and a source gas supply means are arranged in a reaction vessel, and an electric field is applied across the source gas supply means and the substrate, thereby improving uniformity of the film thickness and electric characteristics.
The present inventor found probability that the performance of a deposited film is improved by applying a high-frequency power across the source gas supply means and the substrate in the deposited film forming apparatus described above, and examined this probability. In this case, although electric performance was improved, structural defects do not decrease in number. As a result, to further improve the total performance of the deposited film, some points to be improved are remained.
In particular, in a recent electrophotographic apparatus, not only a character image but also a photographic image must be faithfully copied, and further improvement in image characteristics such as improvement in reproducibility of an intermediate density image is demanded. For this purpose, a faithful latent image is formed on the basis of an original image, and the latent image is developed by using a developer having a small grain size, so as to improve the resolution and reproducibility of the electrophotographic apparatus. As a result, it is required for a deposited film serving as, e.g., a light-receiving member for electrophotography that stable formation of a high dark portion potential, further improvement in electric characteristics having sensitivity which accurately reacts with image exposure, and a reduction in the number of image defects corresponding to improvement in resolution of electrophotographic apparatus, i.e., a reduction in the number of structural defects which cause image defects, are called globular projections, are carried out. In the deposited film forming apparatus described above, there are some points to be improved in formation of a deposited film satisfying the demand of further improvement in total performance of the deposited film.